The present invention relates generally to semiconductor device processing and more particularly to methods for selectively inducing stress in PMOS and NMOS transistors in the manufacture of semiconductor devices.
As semiconductor device switching speeds continue to increase and operating voltage levels continue to decrease, the performance of MOS and other types of transistors needs to be correspondingly improved. The carrier mobility in a MOS transistor has a significant impact on power consumption and switching performance, where improvement in carrier mobility allows faster switching speeds. The carrier mobility is a measure of the average speed of a carrier (e.g., holes or electrons) in a given semiconductor, given by the average drift velocity of the carrier per unit electric field. Improving carrier mobility can improve the switching speed of a MOS transistor, as well as allow operation at lower voltages.
One way of improving carrier mobility involves reducing the channel length and gate dielectric thickness in order to improve current drive and switching performance. However, this approach may increase gate tunneling current, which in turn degrades the performance of the device by increasing off state leakage. In addition, decreasing gate length generally calls for more complicated and costly lithography processing methods and systems.
Other attempts at improving carrier mobility include deposition of silicon/germanium alloy layers between upper and lower silicon layers under compressive stress, which enhances hole carrier mobility in a channel region. However, such buried silicon/germanium channel layer devices have shortcomings, including increased alloy scattering in the channel region that degrades electron mobility, a lack of favorable conduction band offset which may even mitigate the enhancement of electron mobility, and the need for large germanium concentrations to produce strain and thus enhanced mobility.
The use of these additional alloy and silicon layers, moreover, add further processing steps and complexity to the manufacturing process. Furthermore, in CMOS devices, the stress imparted by such a buried silicon/germanium channel layer may adversely affect one type of transistor while improving another. Thus, there remains a need for methods by which the carrier mobility of both NMOS and PMOS transistors may be improved so as to facilitate improved switching speed and low-power, low-voltage operation of CMOS devices, without significantly adding to the cost or complexity of the manufacturing process.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Rather, the primary purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
One aspect of the invention involves methods for fabricating semiconductor devices, in which a tensile film is formed over PMOS transistors to cause a compressive stress therein and a compressive film is formed over NMOS transistors to achieve a tensile device stress. The inventors have found that compressive stress in PMOS device channel regions improves carrier mobility, and further that tensile stress in the channel regions of NMOS transistors improves the carrier mobility thereof. The invention may thus be employed to facilitate improved carrier mobility in both PMOS and NMOS devices.
In one implementation, first and second nitride layers are formed over the PMOS and NMOS transistors using first and second plasma-enhanced chemical vapor deposition (PECVD) processes, respectively. The first deposition provides a tensile nitride film to impart a compressive stress in the channel region of the PMOS device, in turn, increasing the PMOS carrier mobility. The tensile (first) film is removed from over the NMOS device, and the second deposition then provides a compressive nitride film over the NMOS transistor. This compressive film is removed from over the PMOS device, but remains over the NMOS so as to induce a tensile stress in the NMOS channel region. The tensile stress in the NMOS channel region improves the carrier mobility thereof Thus, the invention may be successfully employed to improve carrier mobility in both NMOS and PMOS transistors, for instance, in a CMOS device manufacturing process.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.